1. Field of the Invention
The present invention relates to high power semiconductor laser device used for a light source of an Erbium Doped Fiber Amplifier (EDFA) and, more particularly to high power semiconductor laser device and method for fabricating the same utilizing ion implanting process.
2. Description of the Conventional Art
A semiconductor laser at a 0.98 micro-meter wavelength is used as a light source for the Erbium Doped Fiber Amplifier EDFA for amplifying an optical signal passing through an optical fiber. As an optical output from the above-mentioned semiconductor laser is increased, a light amplifying rate of EDFA becomes higher accordingly. To this end, it is highly demanded to fabricate the 0.98 micro-meter semiconductor laser, which is capable of providing high power output. Especially, it should be apparent that the semiconductor laser for use in the EDFA is required to further improve its optical output, and an optical coupling efficiency between the semiconductor laser and an optical fiber coupled thereto has to be improved for use in module. There has been greatly improved the powerful and reliable 0.98 micro-meter semiconductor laser and the high optical coupling efficiency between the high power semiconductor laser and the optical fiber.
A desired 0.98 micro-meter semiconductor laser module may be made through the coupling of high power semiconductor laser to the optical fiber. However, patterns (emission patterns) of optical output emitted from the semiconductor laser constituting the module are varied depending upon varying operation conditions for the semiconductor laser. This causes an amount of light introduced into the optical fiber to be instantly attenuated, resulting in the deteriorated performance of the semiconductor laser module.
FIGS. 1 to 3 illustrate steps in the fabrication of a conventional 0.98 micro-meter ridge waveguide semiconductor laser device.
FIG. 1 schematically shows a sectional view of the structure consisting, in sequence, of a compound semiconductor substrate 1, a GaInAsP graded layer 2 for further assisting electric current flow caused by band gap difference between GaAs and GaInP layers on the substrate 1, a GaInP clad layer 3, a GaInAsP graded layer 4, a GaInAs/GaInAsP active layer 5, a GaInAsP graded layer 6, a GaInP clad layer 7, a GaInAsP graded layer 8, and a GaAs layer 9 for an ohmic contact. These layers have been grown by Metal Organic Vapor Phase Epitaxy (MOVPE).
Then, as shown in FIG. 2, an insulation layer 10 of SiO.sub.2 or Si.sub.3 N.sub.4 is formed over the resultant specimen of FIG. 2, followed by a well-known photolithography process. This process leaves an insulation layer 10 with the active layer having a constant width of 2 to 3 micro-meters. Sequentially, wet etching or dry chemical etching process is further performed until a top surface of the GaInAsP graded layer 6 is exposed, so that ridges are made as shown in FIG. 2. In FIG. 2, the insulation layer 10 is removed through an appropriate etching process, followed by the formation of another insulation layer 10 of Si.sub.3 N.sub.4 or SiO.sub.2 over the whole surface of the resulting structure. Then, electric current injection window is made to the ridge so as to inject electric current through the ridges. The injection is made through the deposited, plated p-side electrode 11 having a thickness of 2 to 3 micro-meters and which can withstand high current. After thinning the substrate until the thickness of an order of 100 micro-meters is obtained, an n-side electrode 12 is formed, by which the conventional 0.98 micro-meter RWG semiconductor layer is completed.
In case electric current is injected into the conventional semiconductor laser fabricated through steps as described above, current injection is made through ridges within a cavity having width of 2 to 3 micro-meter and length of 800 to 1000 micro-meter. Therefore, only the active layer 5 under the ridge can provide an optical gain, by which light is emitted. There are difference between the refractive indices of respective layers arranged in a vertical direction relative to the active layer 5, wherein the indices get smaller in order of the active layer 5, graded layers 6, 4 and clad layers 7, 3. Further, the difference between the effective refractive index of area under the ridge and index of other area except said area exists, as seen in horizontal direction relative to the active layer 5. With these differences, the light emitted by the above-mentioned gain can be collected around the active layer.
The active layer employed in the RWG semiconductor layer is configured depending upon both gain-guided and index-guided waveguiding properties. When injecting electric current, the active layer of a rectangular stripe shape having been formed through etching process is slightly enlarged in the horizontal direction by current spreading. A length of the active layer corresponds to that of the cavity for the RWG semiconductor laser, while the rectangular width is slightly larger than that of the ridge. In order to provide high output power from the 0.98 micro-meter semiconductor laser, width of the ridge is given as large as possible within a range wherein a sectional area (rectangle) of the active layer is sized to maintain a single lateral mode. The effects of significant lateral spatial hole burning combined with waveguides inherently sensitive to perturbations by injected carriers are manifested in the widespread observation that as they are driven to higher currents, most high power lasers eventually lase on multiple lateral modes. This broken condition causes filament having width of 1 micro-meter and length of 100 to 150 micro-meter to be generated at random. The generation of such filaments is due to the attenuation of a fundamental lateral mode caused by the spatial hole burning phenomenon, and is further due to oscillation of higher-order lateral mode which has higher gain at a side along the cavity axis of the active layer. Lasing mode field pattern of the optical output from the semiconductor laser becomes varied accordingly.
The central axle of emission pattern in the fundamental lateral mode is consistent with that of the cavity. However, in case the filaments occur at side along the cavity axis of the active layer, a beam steering phenomenon is induced which the central axle of emission pattern is deviated from the axle of the cavity to a side opposed to the generated filament. The 0.98 micro-meter semiconductor laser module to which the optical fiber optically aligned under the fundamental lateral mode operation is attached intends to induce varying amount of the light to be coupled to the optical fiber, because of the beam steering phenomenon due to the generated filament. This makes maximum optical output varied and deteriorates the stability of the optical output, resulting in the reduction of performance of the semiconductor laser module.
In other words, injection of high current to the conventional semiconductor laser as mentioned above causes the fundamental lateral mode to be attenuated due to the spatial hole burning. On the other hand, under the above condition, higher-order lateral mode obtains high gain at a side along the cavity axis of the active layer. For these reasons, filaments of 1 micro-meter in width are generated at random. Such filaments causes emission pattern of an optical output at the output surface to be varied. Although width of the active layer may be narrowed to eliminate such a phenomenon, the narrowed active layer provides the low optical power and readily induces damage to the optical output surface.